Horseshow Vortices at Erodible Boundaries of Nonuniform Flows

The results of experimental studies of the interaction between the horseshoe vortices formed in nonuniform water flows and a sand surface are presented. The central part of the initial cylindrical vortex ascends, driven by the Kutta—Joukowski force. The vortex tails submerged into sand approach each other, grabbing the sand by their ends. Sharp bends are formed at the axes of the vortex tails. If the bends occlude, a ring vortex is formed above the bends. The ring approaches the surface at an angle of 40° and moves along the flow: the angle decreases, and the radius of the ring increases. When the whole vortex reaches the water surface, it breaks, loses the entrapped sand, and forms a ridge on the bottom.